A protein called Mrp4 blocks the access of the anti-cancer drug topotecan into the brain by transporting this agent back into the bloodstream, thus reducing the ability of this agent to reach tumors.
Results from a series of studies by investigators at St. Jude Children’s Research Hospital are published in a recent issue of Molecular and Cellular Biology (MCB).
The St. Jude team, which developed a mouse model lacking the Mrp4 protein, says study results in both mice and tissue cultures suggest that the therapeutic efficacy of drugs targeting central nervous system tumors might be improved by inhibiting this protein, a type of molecule called an ABC-dependent transporter.
The study showed that Mrp4 works at two levels: by binding to topotecan and transporting it away from the brain Mrp4 restricts the drug’s penetration into the brain from the bloodstream; and it protects brain cells from accumulating toxic levels of topotecan molecules that do escape the bloodstream.
“The ability of Mrp4 to protect the brain from toxins can be a liability in people with brain cancer when this protein also blocks therapeutic drugs from reaching CNS tumors,” said John Schuetz, Ph.D., an associate member of the St. Jude Department of Pharmaceutical Sciences. Schuetz is senior author of the article.
The investigators discovered that when topotecan was injected into the veins of specially bred mice that lack Mrp4, the drug accumulated to greater than normal levels in the brain tissue and the fluid that surrounds the brain—the cerebrospinal fluid (CSF).
The finding strongly suggests that the natural role of Mrp4 is to block the passage of certain toxic molecules, which chemically resemble topotecan, from leaving the bloodstream and entering the brain. The cells lining the walls of brain capillaries are tightly joined to form a barrier that prevents most substances from leaving the blood. This cellular barrier, called the blood-brain barrier, prevents certain substances from leaving the bloodstream and entering the brain. Mrp4 in the blood-brain barrier also prevents substances from entering the brain by transporting them back into the blood as they pass into the cells of this barrier.
Using antibodies against Mrp4 the investigators found that this protein is located in the brain’s capillaries as well as in membranes of the choroid plexus—the folds within the brain ventricles that make and release CSF.
“This dual location for Mrp4 is unusual for this type of transporter,” Schuetz said. “It suggests that Mrp4 blocks specific molecules from leaving the capillaries. And if such molecules slip out of the blood into the choroid plexus, Mrp4 shuttles them back out of the brain and into the blood before they can cause damage.”
The investigators also showed that isolated cells that were modified to over-express Mrp4 did not accumulate as much topotecan as cells lacking this protein. This is strong evidence that over-expression of Mrp4 in tumors contributes to topotecan resistance in patients.
“Our work has important implications for therapies that target brain tumors with specific types of drugs that are transported by Mrp4,” Schuetz said. “There is an expanding array of these types of drugs being developed; and unless there is a way to block Mrp4 when giving these agents, the effectiveness of these new agents could be significantly compromised.”
Other authors of this study are Markos Leggas, Masashi Adachi, Daxi Sun, Guoqing Du, Kelly E. Mercer, Yanli Zhuang, John C. Panetta, Brad Johnston and Clinton F. Stewart (St. Jude); George L. Scheffer and Rik J. Scheper (VU Medical Center, Amsterdam, The Netherlands); and Peter Wielinga (The Netherlands Cancer Institute, Amsterdam).
This work was supported in part by NIH, a Cancer Center Support Grant, the Dutch Cancer Society and ALSAC.